EPR oximetry of a rat heart in vivo While considerable useful data have been obtained using the isolated heart, there are many aspects which require studies in vivo. These have not been able to be done readily with techniques such as EPR because of the problems of motion and the need to maintain controlled physiological conditions while the animal is within the spectrometer. We now, however, have competed the initial technical developments and obtained useful data on the pO2 within a functioning heart. We used male Wistar rats weighing 200-250 g. Animals initially were anesthetized with a low dose of pentobarbital. and then a tube was placed and positive pressure ventilation started with isoflurane (1%) as the anesthetic. A polyethylene catheter was placed in the right carotid artery for continuos monitoring of blood pressure and periodic blood gas measurements of pO2 (as an experimental parameter) and pCO2 (to maintain it in the physiological range). There was a significant decrease in blood pO2 after the chest was opened. To keep blood pO2 in the physiological range the oxygen concentration in the inspiring gas mixture needed to be increased to 27%. LiPc crystals were placed directly into the middle of the left myocardium. An external loop resonator with a special plastic holder was developed for placement near the heart. This holder included a feature to support the heart at the same position in the external loop resonator. The preliminary data show that the myocardial pO2 = 14.2q2 mm Hg if oxygen tension in blood is in the range 90-95 mm Hg (in group of five animals). After this control measurement the left coronary artery was compressed. In two hearts myocardial pO2 dropped down to zero while in three others there was only a moderate decrease in myocardial pO2. Our preliminary interpretation is that the coronary artery compression causes a very local ischemia and crystals were not within the ischemic area in these three experiments. Reperfusion after 20 minutes compression was carried out in two hearts and these both had a full recovery to the control myocardial pO2 within 20 minutes. The technical capability to do the experiments seems reasonably established.